What Is a Primary Brain Tumor?

Image of brain tumor

Primary brain tumors arise from the brain, spinal cord, and associated tissues—collectively known as the central nervous system—rather than originating from cancerous cells that travel from elsewhere in the body. Primary brain tumors are categorized as either benign (noncancerous) or malignant (cancerous).

Despite their often slow growth, noncancerous primary brain tumors still can have devastating physical and emotional effects. Meningiomas, for example, can grow anywhere on the meninges (protective layers that cover the brain and spinal chord) and as a result can gravely affect areas of the brain needed for such functions as sight, hearing, touch, balance, memory, and judgment.

Research into the causes of brain tumors currently includes investigating environmental factors, exposure to certain viruses, and the role of genes. For most patients, there generally is no obvious exposure or risk factor that can be linked to these tumors.

"Tumor" is a general term describing new growth of cells in an inappropriate manner (serving no useful purpose). The National Cancer Institute defines a tumor as an abnormal mass of tissue that results from excessive cell division. They may be benign (not cancerous) or malignant (cancerous). Brain tumors are tumors of the central nervous system (CNS), which includes the brain, spinal cord, and associated tissue.

There are two types of brain tumors, primary and secondary. Primary brain tumors grow from cells that are part of the central nervous system. Primary brain tumors may be either benign (non-cancerous) or malignant (cancerous). Secondary brain tumors are cancerous tumors that arise from cancer cells that travel from other parts of the body and invade the brain.

Most primary tumors of the central nervous system are named according to the specific type of cells in the nervous system from which they arise. The most common primary brain tumors in adults are meningiomas (33.8% of all primary brain tumors), generally nonmalignant tumors that arise from the membrane that lines the skull and covers the nervous system, and malignant gliomas (32% of all primary brain tumors), which arise from cells that support neurons.

Scientists do not know the exact causes of brain tumors. There are some risk factors that scientists believe may contribute to the development of brain tumors. For primary brain tumors, two environmental risk factors are exposure of the head to Xrays and a history of disorders of the immune system. Certain genetic disorders present risk factors for specific types of brain tumors, both cancerous and noncancerous.

The word “benign” is misleading, implying little danger to the patient. In fact, benign tumors of the central nervous system often cause very serious disability and even death for two main reasons. First, the brain is enclosed in the skull, which cannot expand to make room for a growing tumor, so the tumor may compress and damage delicate brain structures. For example, benign tumors can grow on or near the optic nerve, affecting vision. Some benign brain tumors can change over time, becoming malignant. Because of the potentially serious problems associated with noncancerous brain tumors they are often described as being "high-grade" (rapidly growing) or "low-grade" (slowly growing), rather than malignant or benign.